LIGHT A FOBM OP FORCE. 203 



a vibratory, or, as one of them expi-esses it, a kind of shiv- 

 ering motion of the particles of matter. But perhaps they 

 are not more generally agreed in accepting this view now 

 than they were in holding to the belief that these several 

 principles were so many distinct material substances half 

 a century ago. 



But, whatever the truth may be in regard to the theory, 

 there is no doubt of the fact that light is a form of foi-ce, 

 which is transmitted from the sun or other luminous cen- 

 tre in combination or connection with heat and with actin- 

 ism as the chemical force is called in apparently simple 

 and homogeneous rays, which, however, in consequence of 

 their different degrees of refrangibility, may be separated 

 from each other, the ray of light itself being resolvable 

 into seven or more component portions, which have re- 

 spectively the power of producing in us sensations of the 

 several colors. When any one of these rays enter the eye 

 directly from the prism which has separated them, it pro- 

 duces at once and directly the appropriate sensation. If 

 the spectrum falls upon a screen, or upon any surface serv- 

 ing as a screen, the several portions of it are reflected, and, 

 entering the eye, they produce each its own proper sensa- 

 tions in this secondary manner. 



The rays of light, besides being separable into their com- 

 ponent portions in this way by refraction in passing from 

 one transparent medium into another, are also subject to 

 a somewhat similar modification similar, at least, in some 

 respects when falling upon any opaque substances. Some 

 such substances absorb the whole of the light, and reflect 

 none of it to the eye ; these are black substances. Others 

 reflect the whole, though in a peculiar manner ; these are 

 white substances. Others absorb certain portions of the 

 separated rays and reflect others of them, as the green 

 rays, or the blue rays, for example that is, the rays that 



